The present invention relates to a hydraulic fracturing fluid comprising a block copolymer containing at least one water-soluble block and one hydrophobic block.
It is well known that production in petroleum, natural gas and geothermal wells can be greatly enhanced by hydraulic fracturing techniques. These techniques are known in the art and generally comprise introducing an aqueous solution of a water-soluble polymer (e.g. Guar Gum) in which xe2x80x9cproppantsxe2x80x9d (e.g. coarse sand or sintered bauxite or synthetic ceramic materials) are suspended, through the well bore under extremely high pressures into the rock structure in which the petroleum, gas or steam is entrained. Minute fissures in the rock are thereby created and held open by the suspended particles after the liquid has drained off. The petroleum, gas or steam can then flow through the porous zone into the well. Examples of art relative to to fracturing techniques are U.S. Pat. Nos. 6,169,058, 3,974,077, and 3,888,312.
Polysaccharides, e.g. guar and guar derivatives, are the most commonly used water-soluble polymers for hydraulic fracturing. Examples of art relative to guar are U.S. Pat. Nos. 5,697,444, 5,305,837, and 5,271,466. Viscoelastic gels are formed by the chemical linking or cross-linking of the guar polymer chains. The result is a more ordered network structure which increases the effective molecular weight and thereby, the viscosity(1). Surfactants and synthetic polymers have also been described in patent literature and are employed as gelling agents in fracturing fluids, when present in sufficient concentration to produce stable high viscosity viscoelastic gels. Examples of art relative to surfactants and synthetic polymers are U.S. Pat. Nos. 5,551,516, 6,013,185 6,004,466 and International Pat. WO/056497.
The viscosity stability of the various water-soluble polymer solutions, as a function of time and temperature, is crucial for successful hydraulic fracturing applications in the oil field area. They must retain sufficient suspension properties to deliver and place the proppant effectively to the targeted propagating fissure area, under typical high pressure and temperature conditions which are present down hole in the well bore. The fracturing process itself is relatively short lived, running typically from a few hours, but sometimes for a longer period in severe cases.
There are other important characteristics which must be met for a successful fracturing operation. The fracturing fluid must maintain sufficient proppant suspension and transport properties under a complex combination of rheological conditions presented by pressure, shear rate and temperature changes as the fracturing fluid is transported from the mixing stage, down through the well bore and into the propagating fractured rock fissures. It is necessary that the fluid exhibit predictable shear thinning and friction reduction properties in order the effectively transport it long distances down through the well bore at higher shear. Once in the propagating fissure area the fluid will encounter low shear under high pressure and temperature conditions. This is where its suspension properties are critical to ensure proper proppant packing within the fissure, with minimal dropout or settling of the proppant, which could cause an ineffective fracture once the pressure is released.
In addition to the Theological challenges under varying shear, pressure and temperature, the fluid is also exposed to a variety of chemical conditions which are dependent on the formation composition at the particular well site. These include pH (acidic or alkaline materials), brine (salt content), hardness (mineral content), crude oil and natural gas variations, which the fluid will contact and absorb as it travels through the propagating fracture.
The last major consideration is formation damage, once the fractured fissure has been completed and the proppant is properly placed. Great effort is made to remove as much of the gel-fluid component as possible, in order to reduce the amount of residue (polymeric or surfactant) left behind in the formation and proppant channel areas. A portion of it will naturally xe2x80x9cleak-offxe2x80x9d into the surrounding formation during the fracture process. This residue can reduce the effectiveness of the efficiencies gained by the fracturing process, by creating blockages in the minute porosity present in the fractured fissure face and channel. This is generally known as formation damage. It is desirable to minimize this condition in order to maximize the well production gain from the fracturing process. It is common to employ enzymatic or chemical xe2x80x9cbreakersxe2x80x9d (example of that art: U.S. Pat. No. 5,697,444) when working with guar gels to depolymerize the polymer, which lowers the viscosity and allows a higher recovery of the fluid back through the well bore, although an amount of residual polymer is left behind. Surfactant systems most commonly known, rely on contact with the formation hydrocarbon (crude oil or liquefied gas) as a natural breaking mechanism to lower viscosity. Claims are made that this type of system leaves virtually no residue behind in the formation. The Industry generally refers to the term xe2x80x9cpermeabilityxe2x80x9d to describe relative formation damage. A value of 100% return-permeability would denote that the formation permeability is equal to its original value, before exposure to a fracturing polymer or surfactant. A lower number would denote a reduction in formation permeability (formation damage) caused by polymer residue. In practice, the best traditional guar system cleanup will result in a maximum of 75% permeability, while surfactant systems claim values of 95-100%. The industry also uses the term xe2x80x9cconductivityxe2x80x9d to denote the relative permeability of the proppant filled fissure area, following a fracture job
One of the essential objectives of the present invention is to provide a fracturing fluid whose viscosity is stable as high temperatures as 190xc2x0 C., preferably 200xc2x0 C. and even higher.
Another objective of the invention is to provide a fracturing fluid with sufficient proppant suspension and transport properties.
Another objective of the invention is to provide a fracturing fluid causing no or almost no formation damage.
Another objective of the invention is to provide a fracturing fluid still efficient when exposed to a variety of chemical conditions.
These objectives and others which will appear subsequently, are attained by means of a hydraulic fracturing fluid comprising a block copolymer containing at least one block water-soluble in nature and at least one block predominantly hydrophobic in nature. More precisely, the invention relates to a hydraulic fracturing fluid composition comprising:
a) an aqueous liquid, and
b) a block copolymer comprising at least one block water-soluble in nature and containing hydrophobic units and at least one block predominantly hydrophobic in nature forming a viscoelastic gel in said aqueous liquid.
According to a first embodiment, the copolymer contains only a single hydrophobic block and a single water-soluble block. According to another embodiment, the copolymer contains a water-soluble block having a hydrophobic group at each end or the copolymer contains a hydrophobic block having a water-soluble group at each end.
In the description which follows, the expression xe2x80x9cblock water-soluble in naturexe2x80x9d should be understood to mean a polymer block containing a number of hydrophilic groups sufficient to obtain a water soluble block well dissolved in water. Solubility in water of the water soluble block means a block copolymer containing such a water soluble block, when mixed with water, gives a translucent monophasic system. Usually such a translucent monophasic system is obtained from a water soluble block comprising at least 30%, preferably at least 50% by weight of hydrophilic units with respect to the totality of units of the water-soluble block. The block water-soluble in nature is therefore soluble in water. The term xe2x80x9cunitxe2x80x9d should be understood to mean that part of the block corresponding to a monomeric unit.
Likewise, the expression xe2x80x9cblock predominantly hydrophobic in naturexe2x80x9d should be understood to mean a polymer block preferably containing at least 67% by weight hydrophobic units with respect to the totality of units. The block predominantly hydrophobic in nature is not soluble in water. This block copolymer containing at least one block water-soluble in nature and at least one block predominantly hydrophobic in nature forms a viscoelastic gel when it is in solution in water.
The term xe2x80x9cviscoelastic gelxe2x80x9d should be understood to mean a liquid medium for which the viscous modulus Gxe2x80x3 and the elastic modulus Gxe2x80x2 are such that Gxe2x80x2 greater than Gxe2x80x3. This gel behaviour is manifested by a flow threshold and even, in some cases, by a shear-thickening effect (an increase in the viscosity with flow). This gel effect is obtained when the polymer concentration exceeds a certain threshold called the critical gelling concentration.
The block copolymers according to the present invention have the advantage of making the aqueous media viscoelastic when they are used in only a small amount with respect to the aqueous medium. The copolymer may be used at in said hydraulic fluid at a concentration higher than 0.1% by weight, more particularly between 0.5 and 10% by weight and even more preferably at a concentration from 1 to 5% by weight.
The appropriate viscoelastic properties of the copolymers according to the present invention may be obtained by selecting the nature of the soluble blocks and the nature of the predominantly hydrophobic blocks, at least the hydrophilic block having to contain hydrophobic groups in an appropriate amount.
According to one embodiment of the invention, the weight ratio of the block water-soluble in nature to the completely hydrophobic block is between 95/5 and 20/80, even more preferably between 90/10 and 40/60.
According to a first version of the preparation, the blocks water-soluble in nature and the blocks predominantly hydrophobic in nature of the above copolymers may come from the copolymerization of hydrophilic and hydrophobic monomers. The amounts of hydrophilic and hydrophobic units in each of said blocks can then be controlled by the respective contents of hydrophilic monomers and hydrophobic monomers during the polymerization of the blocks.
Thus, the blocks predominantly hydrophobic in nature may come from the copolymerization of hydrophobic monomers and of hydrophilic monomers, the hydrophilic monomers being present in an amount of less than 33% by weight, preferably at least 1% by weight, even more preferably between 2 and 15%, with respect to the total weight of the units of the hydrophobic block.
In addition, the blocks water-soluble in nature may come from the copolymerization of hydrophilic monomers and of hydrophobic monomers, the hydrophobic monomers being present in an amount of less than 70% by weight, preferably at least 1% by weight, even more preferably between 50 and 10%, with respect to the total weight of the units of the water-soluble block.
According to a second version of the preparation, the blocks water-soluble in nature may come:
from the polymerization of monomers that may be rendered hydrophilic by hydrolysis and optionally of non-hydrolysable hydrophobic monomers and/or of hydrophilic monomers, and then
from the hydrolysis of the polymer obtained.
During the hydrolysis, the units corresponding to the hydrolysable monomers are hydrolysed into hydrophilic units.
The amounts of hydrophilic and hydrophobic units in each of said blocks are then controlled by the amount of each type of monomer and by the degree of hydrolysis.
According to this second version, various methods of implementation may be envisaged.
According to a first method of implementation, the blocks may be obtained by:
homopolymerization of hydrophobic monomers that can be rendered hydrophilic by hydrolysis and
partial hydrolysis of the homopolymer obtained to a degree such that what is obtained is:
either, in the case of the blocks predominantly hydrophobic in nature, an amount of hydrophilic units of less than 33% by weight, preferably at least 1% by weight, even more preferably between 2 and 15%, with respect to the total weight of the units of the hydrophobic block,
or, in the case of the blocks water-soluble in nature, an amount of hydrophobic units of less than 70% by weight, preferably at least 1% by weight, even more preferably between 25 and 50%, with respect to the total weight of the units of the water-soluble block.
According to a second method of implementation, the blocks may be obtained by:
copolymerization of hydrophobic monomers that can be rendered hydrophilic by hydrolysis and of hydrophobic monomers that cannot be rendered hydrophilic by hydrolysis and then
complete or partial hydrolysis of the polymer obtained.
According to this second method of implementation, the amount of hydrophilic and hydrophobic units may depend on two criteria, namely the content of the various types of monomers and the degree of hydrolysis.
If there is complete hydrolysis, it is sufficient to vary the content of the monomers and thus:
the blocks predominantly hydrophobic in nature can come:
from the polymerization of a mixture of hydrophobic monomers that can be rendered hydrophilic by hydrolysis and of hydrophobic monomers that cannot be rendered hydrophilic by hydrolysis, the hydrophobic monomers that can be rendered hydrophilic by hydrolysis being present in an amount of less than 33% by weight, preferably at least 1% by weight, even more preferably between 2 and 15%, with respect to the total weight of the units of the hydrophobic block, and then,
from the complete hydrolysis of the polymer obtained;
the blocks water-soluble in nature may come:
from the polymerization of a mixture of hydrophobic monomers that can be rendered hydrophilic by hydrolysis and of hydrophobic monomers that cannot be rendered hydrophilic by hydrolysis, the hydrophobic monomers that cannot be rendered hydrophilic by hydrolysis being present in an amount of less than 50% by weight, preferably at least 1% by weight, even more preferably between 49 and 10%, with respect to the total weight of the units of the hydrophobic block, and then
from the complete hydrolysis of the polymer obtained.
If there is partial hydrolysis, the monomer content and the degree of hydrolysis may be varied at the same time.
According to a third method of implementation, the blocks may be obtained by:
copolymerization of hydrophobic monomers that can be rendered hydrophilic by hydrolysis and of hydrophilic monomers and then
partial hydrolysis of the polymer obtained to a degree such that what is obtained is:
either, in the case of the blocks predominantly hydrophobic in nature, an amount of hydrophilic units of less than 33% by weight, preferably at least 1% by weight, even more preferably between 2 and 15%, with respect to the total weight of the units of the hydrophobic block,
or, in the case of the blocks water-soluble in nature, an amount of hydrophobic units of less than 70% by weight, preferably at least 1% by weight, even more preferably between 50 and 10%, with respect to the total weight of the units of the water-soluble block.
In general, the hydrophobic monomers may be chosen from:
vinylaromatic monomers, such as styrene,
dienes, such as butadiene,
alkyl acrylates and methacrylates the alkyl group of which contains from 1 to 10 carbon atoms, such as methyl, ethyl, n-butyl, 2-ethylhexyl, tert-butyl, isobornyl, phenyl and benzyl acrylates and methacrylates.
Preferably, it is styrene.
The hydrophilic monomers may be chosen from:
ethylenically unsaturated carboxylic acids such as acrylic and methacrylic acids;
neutral hydrophilic monomers such as acrylamide and its derivatives (N-methylacrylamide, N-isopropylacrylamide), methacrylamide, polyethylene glycol methacrylate and polyethylene glycol acrylate;
anionic hydrophilic monomers: sodium 2-acrylamido-2-methylpropanesulphonate (SAMPS), sodium styrenesulphonate and sodium vinylsulphonate.
The monomers that can be rendered hydrophilic by hydrolysis may be chosen from:
acrylic and methacrylic esters hydrolysable in acid, such as methyl acrylate, ethyl acrylate, hydroxyethyl methacrylate, hydroxyethyl acrylate and tert-butyl acrylate;
vinyl acetate hydrolysable into vinyl alcohol units;
quaternized 2-dimethylaminoethyl methacrylate and acrylate (quatdamma and quatdama);
acrylamide and (meth)acrylamide.
Preferably, the block copolymers according to the invention are diblock copolymers. However, they may also be triblock, or even multiblock copolymers. If the copolymer comprises three blocks, it is preferable to have a block water-soluble in nature flanked by two blocks predominantly hydrophobic in nature.
According to a particular embodiment of the invention, the copolymer is a diblock copolymer comprising a block water-soluble in nature and a block predominantly hydrophobic in nature, in which:
the block water-soluble in nature contains acrylic acid (AA) units and ethyl acrylate (EtA) units and
the block predominantly hydrophobic in nature contains styrene (St) units and methacrylic acid (MAA) and/or hydroxyethyl methacrylate (HEMA) units.
Preferably, according to this embodiment, the block water-soluble in nature comes:
from the polymerization of methacrylic acid (MAA) and of ethyl acrylate (EtA) in an EtA/MAA weight ratio from 90/10 to 99/1, and then
from the hydrolysis of the polymer obtained to a degree of at least 50 mol % up to 95%.
Preferably, the block predominantly hydrophobic in nature comes from the polymerization of a monomer mixture comprising at least 80% by weight styrene.
Generally, the block copolymers according to the invention have a molecular mass of at most 100,000 g/mol, preferably at least 1000 g/mol.
In general, the above block copolymers can be obtained by any so-called living or controlled polymerization process such as, for example:
radical polymerization controlled by xanthates according to the teaching of Application WO 98/58974,
radical polymerization controlled by dithioesters according to the teaching of Application WO 97/01478,
polymerization using nitroxide precursors according to the teaching of Application WO 99/03894,
radical polymerization controlled by dithiocarbamates according to the teaching of Application WO 99/31144,
atom transfer radical polymerization (ATRP) according to the teaching of Application WO 96/30421,
radical polymerization controlled by iniferters according to the teaching of Otu et al., Makromol. Chem. Rapid. Commun., 3, 127 (1982),
radical polymerization controlled by degenerative iodine transfer according to the teaching of Tatemoto et al., Jap. 50, 127, 991 (1975), Daikin Kogyo Co Ltd., Japan and Matyjaszewski et al., Macromolecules, 28, 2093 (1995),
group transfer polymerization according to the teaching of O. W. Webster xe2x80x9cGroup Transfer Polymerizationxe2x80x9d, pp. 580-588 in xe2x80x9cEncyclopedia of Polymer Science and Engineeringxe2x80x9d, vol. 7 and H. F. Mark, N. M. Bikales, C. G. Overberger and G. Menges, Publ., Wiley Interscience, New York, 1987,
radical polymerization controlled by tetraphenylethane derivatives (D. Braun et al., Macromol.Symp. 111,63 (1996)), and
radical polymerization controlled by organocobalt complexes (Wayland et al., J.Am.Chem.Soc. 116,7973 (1994)).
The preferred polymerization is living radical polymerization using xanthates.
A possible process for preparing these block copolymers consists in:
1 the following being brought into contact with one another:
at least one ethylenically unsaturated monomer,
at least one source of free radicals and
at least one compound of formula (1): 
xe2x80x83wherein:
R represents an R2Oxe2x80x94, R2Rxe2x80x22Nxe2x80x94 or R3xe2x80x94 group, where:
R2 and Rxe2x80x22, which are identical or different, represent (i) an alkyl, acyl, aryl, alkene or alkyne group or (ii) a saturated or unsaturated, possibly aromatic, carbocycle or (iii) a saturated or unsaturated heterocycle, these groups and rings (i), (ii) and (iii) possibly being substituted,
R3 represents H, Cl, an alkyl, aryl, alkene or alkyne group, a saturated or unsaturated, optionally substituted (hetero) cycle, an alkylthio, alkoxycarbonyl, aryloxycarbonyl, carboxy, acyloxy, carbamoyl, cyano, dialkylphosphonato, diarylphosphonato, dialkylphosphinato or diarylphosphinato group, or a polymer chain,
R1 represents (i) an optionally substituted alkyl, acyl, aryl, alkene or alkyne group or (ii) an optionally substituted or aromatic, saturated or unsaturated, carbocycle or (iii) an optionally substituted, saturated or unsaturated, heterocycle, or a polymer chain;
2 the above contacting operation being repeated at least once, using:
monomers differing from those in the previous operation, and
instead of the precursor compound of formula (I), the polymer coming from the previous operation; and
3 optionally, the copolymer obtained being hydrolysed.
The R1, R2, Rxe2x80x22 and R3 groups may be substituted with alkyl groups, substituted phenyls, substituted aromatic groups or one of the following groups: oxo, alkoxycarbonyl or aryloxycarbonyl (xe2x80x94COOR), carboxy (xe2x80x94COOH), acyloxy (xe2x80x94O2CR), carbamoyl (xe2x80x94CONR2), cyano (xe2x80x94CN), alkylcarbonyl, alkylarylcarbonyl, arylcarbonyl, arylalkylcarbonyl, isocyanate, phthalimido, maleimido, succinimido, amidino, guanidimo, hydroxyl (xe2x80x94OH), amino (xe2x80x94NR2), halogen, allyl, epoxy, alkoxy (xe2x80x94OR), S-alkyl, S-aryl, silyl, groups having a hydrophilic or ionic character, such as alkali metal salts of carboxylic acids, alkali metal salts of sulphonic acid, polyoxy alkylene (POE, POP) chains, and cationic substituents (quaternary ammonium salts), R representing an alkyl or aryl group.
Preferably, the compound of formula (1) is a dithiocarbonate chosen from compounds of the following formulae (IA), (IB) and (IC): 
wherein:
R2and R2xe2x80x2 represent (i) an alkyl, acyl, aryl, alkene or alkyne group, or (ii) an optionally aromatic, saturated or unsaturated, carbocycle or (iii) a saturated or unsaturated heterocycle, these groups and rings (i), (ii) and (iii) possibly being substituted;
R1 and R1xe2x80x2 represent (i) an optionally substituted alkyl, acyl, aryl, alkene or alkyne group or (ii) an optionally substituted or aromatic, saturated or unsaturated, carbocycle or (iii) an optionally substituted, saturated or unsaturated, heterocycle, or a polymer chain;
p is between 2 and 10.
During step 1, a first block of the copolymer is synthesized so as to become water soluble or hydrophobic in nature depending on the nature and the amount of monomers used. During step 2, the other block of the polymer is synthesized.
The ethylenically unsaturated monomers will be chosen from the hydrophilic, hydrophobic and hydrolysable monomers defined above, in proportions suitable for obtaining a block copolymer whose blocks have the characteristics of the invention. According to this process, if all the successive polymerization steps are carried out in the same reactor, it is generally preferable for all the monomers used during one step to have been consumed before the polymerization of the next step starts, therefore before the new monomers have been introduced. However, it may happen that the hydrophobic or hydrophilic monomers of the previous step are still present in the reactor during the polymerization of the next block. In this case, these monomers generally represent no more than 5 mol % of all the monomers and they participate in the following polymerization by contributing to introducing hydrophobic or hydrophilic units into the next block.
For more details with regard to the above polymerization processes, the reader may refer to the contents of Application WO 98/58974.
The hydrolysis may be carried out using a base or an acid. The base may be chosen from alkali or alkaline-earth metal hydroxides, such as sodium hydroxide or potassium hydroxide, alkali metal alcoholates, such as sodium methylate, sodium ethylate, potassium methylate, potassium ethylate and potassium tert-butylate, ammonia and amines such as triethylamines. The acids may be chosen from sulphuric acid, hydrochloric acid and paratoluenesulphonic acid. It is also possible to use an ion-exchange resin or an ion-exchange membrane of the cationic or anionic type. The hydrolysis is generally carried out a temperature of between 5 and 100xc2x0 C., preferably between 15 and 90xc2x0 C.
After hydrolysis, the block copolymer can be washed, for example by dialysis against water, or using a solvent such as alcohol. It may also be precipitated by lowering the pH below 4.5.
The hydrolysis may be carried out on a monoblock polymer, which will then be linked to other blocks, or on the final block copolymer.
The block copolymer is present in the aqueous liquid of said fracturing fluid at a concentration by weight of between 0.1 and 10%, more preferably between 0.5 and 5%, and even more preferably between 1 and 3% vis-à-vis the total weight of said aqueous liquid. The aqueous liquid of said fracturing fluid comprises water and all other liquid components if any.
The essential component of the fracturing fluid is water which may be deionized or contain ions. Good results have surprisingly occurred when using so-called xe2x80x9chardxe2x80x9d water, which may contain magnesium ions, calcium ions, or sodium ions among other metallic ions. The respective amounts of ions in the water typically range from about 10 to about 50 ppm, about 100 to about 300 ppm sodium ions and about 50 to 150 ppm calcium ions.
Still other additives include proppants which may be provided with the fracturing fluid to maintain the fissures caused by pumping and thickening the fracturing fluid into the well bore. Proppant particles include for example, gravel, quartz sand grains, sintered bauxite, glass and ceramic beads, walnut shell fragments, aluminum pellets and the like. The propping agents are typically included in an amount of 0.2 to 3 kg per liter of fluid and the particle size is about 2 U.S. mesh.
The fracturing fluid may comprise one or more thermal stabilizing agents known in the art for use in connection with fracturing fluids such as sodium thiosulfate, methanol, ethylenglycol, isopropanol, thiourea, and sodium thiosulfite.
The fracturing fluid may further include clay stabilizers, for example KCl, whose concentration by weight in said fluid is typically between 1.0 and 4.0%.
Preparing the fracturing fluid comprises mixing the various components together in the amounts above indicated.
The process of using the fracturing fluid comprising the step of injecting into a well bore at a feed rate, pressure and shear rate necessary to create fissures into the subterranean formation at high temperatures. The fracturing fluid of the present invention typically presents a minimum viscosity of 50 cp, at a shear rate of 40 sxe2x88x921 up to about 210xc2x0 C. during 3 h and outmatches guars and viscoelastic polymers known in the art of fracturing.
The following examples illustrate the invention without however limiting its scope.